Abstract

Characteristics and detailed morphology of the genus Phormidesmis Turicchia et al. 2009, recently described according to a combined molecular and phenotype evaluation, and review of phenotypically similar species from this genetic cluster are presented in this paper. The generic entity of Phormidesmis, based on the tropical type-species Phormidesmis molle (formerly Phormidium molle) has a special morphology, different from the typical Phormidium members, and characteristic for this separate generic cluster (moniliform shape of trichomes, one, rarely two or several trichomes in a sheath, not differentiated terminal cells without calyptra). The genus Phormidesmis is more related to thin pseudanabaenacean cyanobacteria than to Phormidium sensu stricto and this relation was also confirmed by ultrastructural analysis (with mainly parietal arrangement of thylakoids); this genus must be therefore classified taxonomically into the family Pseudanabaenaceae. The Antarctic Phormidium priestleyi, with a morphology similar to Phormidium molle, must be eliminated from the genus Phormidium and belongs also into the family Pseudanabaenaceae according to 16S rRNA gene sequencing and ultrastructure of cells. It was therefore taxonomically transferred also ad interim into the genus Phormidesmis. The other potential and similar morphotypes from the genus Phormidium are discussed.